Lapping valves on a Cb125

[SpaceTrucker]

I’m in the middle of an engine rebuild... first time for me. I’m lapping the valves and it seems to be going fine but I’m not seeing that ring of polished metal on the valve (like I’ve seen on how-to videos). When lapping I get the smooth quiet sound and it begins to squeak at the end of each round. I’ve cleaned and repeated with new paste 5 or 6 times.

Should I continue until I see a substantial polished ring near the outer edge of the valve?

 

[whynot]

Good question, I'd like to know the answer to it also.

I lapped the valves on a 2.4 Mighty Max engine, and got the same type of results.

I just put the engine back together and it runs the same way it did before the tear down.

 

[joelvaughan56]

a very thin polished line will do place then valve back in the head attach the spring and collets then put some brake cleaner or wd 40 or a similar thin liquid in the port then to test how they seal

 

[crazypj]

It should never be polished but will be a matte grey. Squeaking means you've probably already gone to far. What lapping compound are you using?

 

[SpaceTrucker]

Thanks guys. Matte grey is what I achieved. I was using Permatex Valve Grinding Compound. After I originally posted I watched a few videos and felt good about the matte grey. Not sure why I thought I needed a polished look. Anyways, I did a liquid test without even installing the springs and the seal was good.

 

[teazer]

When you lap a valve to the seat, you usually end up with the whole width of the seat a dull matte because the whole width has been lapped.

But that creates a very wide contact patch, so what is done in a machine shop is that the outer edge and inner edges are machined at different angles so that it leaves only a relatively narrow band of matte with a shiny ring/s.

On a race bike that's part of making more power and is not critical on a small capacity low power engine.

Ideally one would end up with say 1mm wide on the inlet valves and say 2mm on exhaust valves. The seats are also cut at 3 or 5 angles to improve flow through the valve seat area.

 

[SpaceTrucker]

Good to know. Those are about the measurements I ended up with. My inlet valve may be a little wider that 1mm, though, but not much. The exhaust is about 1.75mm.

-Parke

 

[teazer]

That will work just fine.